We propose to conduct a comprehensive study of electromagnetic field (EMF) exposures in a large population of radiofrequency (RF) heat sealer operators and monitor changes in melatonin, a proposed biomarker of response to EMF. The working hypothesis of this study is that melatonin levels will decrease in the subjects exposed to RF fields, in a pattern that mimics the dose-dependent decrease in melatonin we have observed previously in utility workers. This study will be the most detailed exposure survey of RF heat sealer workers to date. This study will be the first to use state-of-the-art full-shift personal monitoring to document the diverse exposures to RF and power-frequency EMFs in this worker population. This also will be the first study to examine biomarkers in response to RF fields. This study will make a strong contribution to improving RF exposure assessment using personal dosimetry. We will use task-based exposure assessment with time-activity information to identify tasks that contribute most to exposures. This will help identify important exposure scenarios for possible engineering changes or other controls. We propose to use two novel instruments, one that will characterize the induced foot currents in the operators and the second to measure the electric and magnetic field components. The application of induced body currents for free ranging subjects is a novel approach to RF dosimetry that can extend beyond the present application. Many other RF exposed occupational populations exist that would benefit from improvements in RF exposure assessment, and emerging technologies that use the RF spectrum will increase the potential for worker exposures. Understanding the benefits and limitations of improved personal monitoring methods for RF in a working population will provide new tools for worker exposure assessment in these emerging technologies.